Ionization manometer including means for modulation of terminal grids



Apr1l28, 1970 A. G. J. VAN OOSTROM 3,509,418

IONIZATION MANOMETER INCLUDING MEANS FOR MODULATION OF TERMINAL GRIDS Filed May :3, 1968 5 Sheets-Sheet 1 -a +213vT +2 3v I fi 1 H (-LSV) INVENTOR.

ANTONIUS 6.J. VAN OOSTROM BY M AL? A NT April 28, 1970 A. G. J. VAN OOSTROM 3,509,418

IONIZATION MANOMETER INCLUDING MEANS FOR MODULATION OF TERMINAL GRIDS 5 Sheets-Sheet 2 Filed May 5, 1968 INVENTOR. ANTONIUS G.J.VAN OOSTROM April 28, 1970 A. G. J. VAN OOSTROM 3,509,418

IONIZATION MANOMETER INCLUDING MEANS FOR MODULATION OF TERMINAL GRIDS Filed May 3, 1968 5 Sheets-Sheet 5 ,v (v) a INVENTOR. ANTONIUS GJNAN OOSTROM Apr1l28, 1970 A. G. J. VAN OOSTROM 3,

IONIZATION MANOMETER INCLUDING MEANS FOR MODULATION OF TERMINAL GRIDS Filed May 5. 1968 5 Sheets-Sheet 4 P(Torr) INVENTOR. ANTONIUS (SJ-VAN OOSTROM A ril 28-, 1970 A. c. 'J. VAN OOSTROM 3,509,413

IONIZATION MANQMETER INCLUDING MEANS FOR MODULATION OF TERMINAL GRIDS Filed May 5.- 1968 .5 Sheets-Shet 1O PTTorr) illl illlli 10- Q I e a l a Vm (V) INVENTOR. ANTONIUS G .J .VAN OOSTROM United States Patent 3,509,418 IONIZATION MANOMETER INCLUDING MEANS FOR MODULATION OF TERMINAL GRIDS Antom'us Gerardus Johannes van Oostrom, Emmasingel,

Eindhoven, Netherlands, assignor, by mesne assignments, to U.S. Philips Corporation, New York, N.Y.,

a corporation of Delaware Filed May 3, 1968, Ser. No. 718,102 Int. Cl. G01n 27/62 US. Cl. 315-408 3 Claims ABSTRACT OF THE DISCLOSURE An ionization manometer for measuring low gas pres sure employs a thin wire-shaped electrode stretched inside a grid-shaped anode. A filamentary cathode is positioned outside the anode and the interior of the anode is screened at the ends by terminal grids. A potential modulated with respect to the anode potential is applied to an electrode within the manometer resulting in a modulation factor approaching unity.

The invention relates to an ionization manometer for measuring low gas pressures.

More particularly, the invention relates to an ionization manometer in which a thin, wire-shaped collector electrode is stretched inside a grid-shaped anode and a filament cathode is arranged outside the anode. The interior of the anode is screened at the ends by terminal grids and a voltage modulated with respect to the anode voltage is applied to an electrode in the ionization manometer.

A known ionization manometer of this type comprises a wire-shaped modulator electrode inside a grid-shaped anode, which is integral with the terminal grids, which electrode is arranged parallel to the collector electrode and receives a voltage modulated with respect to the anode voltage which varies between the anode voltage and the collector voltage. Thus, as the modulator voltage approaches the collector voltage, more ions are collected by the modulator electrode, so that the collector current is lower. The collector current variations involved in the modulation permit the elimination of interfering residual currents from the measured results. The residual currents may be formed by photo-emission currents from the collector electrode due to X-rays from the grid-shaped anode struck by electrons, or to ultraviolet radiation from the cathode. Moreover, desorption of molecules adsorbed at the anode may contribute considerably to the residual current, particularly in the presence of 0 or CO. The elimination of residual currents results from the variation of the modulation voltage affecting differently the real ion current, i.e., the flow of ions from the gaseous phase, which determines the real gas pressure, and the residual current. The influence on the residual current is small and may often be neglected as compared with the influence on the real ion fiow. The influence on the real ion flow may be indicated by the modulation factor k of the real ion flow, which factor designates the variation of the ion flow by the modulation. This modulation factor of the real ion flow is substantially independent of the pressure. If the pressure is so high that the real ion flow considerably exceeds the residual current, the modulation factor k of the real ion flow is substantially equal to the 3,509,418 Patented Apr. 28, 1970 collector-current modulation factor k, which designates the variation of the total collector current by modulation. Therefore, k may be determined by measuring in a gas with an ion flow, where said residual effects do not play an appreciable part. The modulation factor k of the real ion flow is a function of the geometry, the applied voltages and the electron flow and it varies slightly with different constructions of this ionization manometer.

In such an ionization manometer the real ion flow (and the real gas pressure) may be determined by the magnitude of the collector-current variation (or the variation of the indicated gas pressure) in the case of modulation. The accuracy of determination of the real ion flow (or the real gas pressure) increases with an increase in relative variation of the collector current (or of the indicated gas pressure) and hence with a higher modulation factor k. This manometer has the disadvantage that the modulation factor k has a relatively low value in spite of the great variation of the voltage applied to the modulator electrode. The modulation factors of these constructions have values lower than 0.5. With gas pressures of the order of 10- torr and lower, desadsorption effects may readily give rise to such a high residual current that because of the small relative variation of the indicated gas pressure, the real gas pressure can no longer be determined With requisite accuracy.

The invention has for its object to provide an ionization manometer operating by means ofmodulation, having a modulation factor of the real ion flow which approaches more nearly the ideal value 1.

According to the invention, in an ionization manometer in which a thin wire-shaped collector electrode is stretched inside a grid-shaped anode and a filament cathode is arranged outside the anode, the space inside the anode being screened at the ends of the anode by terminal grids, and a voltage modulated with respect to the anode voltage is applied to an electrode in the ionization manometer, the modulated voltage is applied to at least one of the terminal grids separated from the anode.

If the instantaneous value of the modulation voltage, that is to say the voltage on the terminal grid separated from the anode minus the anode voltage in this ionization manometer is zero, an ion produced in the gas inside the grid space is subject to a given axial force due to the instantaneous potential distribution. If the modulation voltage has a different value, such an ion is subject to a different axial force, and there is another probability of it escaping the grid space before the radius of its path is sufliciently small for being collected. The variation of the modulation voltage has a considerably smaller influence on electrons and any desorbed ions than on ions produced in the gaseous phase, since the electrons and the desorbed ions have a high initial velocity.

The effect of the modulation on the terminal grid is greater as the collector electrode is made thinner, since in the case of a thinner collector the ions can perform more revolutions around the collector with decreasing radii. The diameter of the collector wire, therefore preferably does not exceed a few microns.

In a particularly advantageous embodiment the collector wire has a diameter not exceeding a few microns and the difference between the anode voltage and the collector voltage is about 3.5 times the difference between the anode voltage and the cathode voltage so that, as is known, a maximum sensitivity is attained, while the modulated voltage is applied to the two terminal grids and themaximum of the absolute value of the modulation voltage is about /6 of the voltage difference between the anode and the collector. Such a low modulation voltage may suffice for obtaining in this embodiment a modulation factor k of the real ion flow of about 0.9, while an increase in modulation voltage has substantially no further effect.

The ionization manometer in which the terminal grids are separated from the anode is known per se. It has furthermore been stated a potential difference between the anode and the terminal grids might improve the sensitivity of the manometer. However, these manometcrs are driven solely by fixed voltages and any possibility of eliminating the residual current is not indicated.

The invention will be described with reference to the accompanying drawing in which FIG. 1 is the major part of a longitudinal sectional view of an ionization manometer tube according to the invention,

FIG. 2 shows the same tube partially in a longitudinal sectional view in a direction at right angles to the sectional view of FIG. 1,

FIGS. 3, 4 and show modulation characteristic curves of the ionization manometer tube of FIGS. 1 and 2.

Referring to FIGS. 1 and 2, reference numeral 1 designates the cylindrical envelope of the manometer tube, having a powdery-glass bottom 2. The tube has a connecting part 3 for the space in which the pressure is to be measured. The through connection pins 4 support the two filament cathodes 5 of tungsten wire. The gridshaped anode 6 of molybdenum wire is supported from molybdenum rods, which are held by the pins 8. The grid space is screened by two terminal grids 9 of molybdenum wire, secured to molybdenum rings 10, held by the pins 11. Two side extensions 12 have glass-covered sealed-in through-connection pins 13, to one of which is secured a spring 14, which stretches the tungsten wire 15 as a collector electrode inside the anode. The ends of the pins 13 are surrounded by quartz tubes 17, provided with leaf springs 16 and extending as far as beyond the welds between the wire 15 and the pins 13 and across the spring 14. A side extension 18 in the connecting part 3 comprises a through-connection pin 19, which is in contact by a spring 20 with a conductive tin oxide layer 21, which extends from the connecting part 3 to near the bottom 2. A wall portion 22 around the through-connection pins 13 is free of i said layer.

In the embodiment the collector electrode 15 has a diameter of 4p. and the length of the portion of the collector'not covered by the quartz tubes 17 is 35 mm.

The grid-shaped anode 6 is made of a wire having a diameter of 120;. The anode has 25 turns and a diameter of 20 mm. and a length of .40 mm.

The molybdenum rods 7 have a diameter of 0.6 mm.

The terminal grids 9 are made of molybdenum wire of a diameter of 120 The distance between the grid wires is 3 mm.

The molybdenum rings have a diameter of 0.6 mm.

Each of the filament cathodes 5 is made from a wire of a diameter of 125;/.. Each of the filament cathodes has 22 turns and an over-all length of 70 mm.

The envelope 1 has a diameter of 60 mm. and a height of 72 mm.

The modulation characteristic curves of FIGS. 3, 4 and 5 relate to an ionization manometer tube shown in FIGS. 1 and 2, in which as indicated (only) in FIGURE 1, the collector potential is 0 v., the cathode potential is +213 v. and the anode potential is +28 8 v., while the conductive tin oxide layer is held at cathode potential.

FIG. 3 shows the modulation characteristic curves for an argon-filled tube. The real argon pressure is 5X10- torr. The indicated pressure is plotted as a function of the modulation voltage V applied to the terminal grids. The voltage V indicates the voltage at the terminal grids minus the anode voltage in volts. The three curves I, II and III are associated with three different values of the electron flow of 1 ma., a. and 10 ,ua. respectively. The characteristic curves exhibit the strong influence of the modulation voltage on the ion flow, which influence is substantially independent of the electron flow. A modulation voltage of an extreme of 45 v., applied between anode 6 and grids 9 from a source 23, shown in FIGURE 1, is suflicient for an appreciable variation of the ion flow. The characteristic curves I, II and III correspond to collector-current modulation factors:

Since in each of these cases the condition is satisfied that the residual current should be negligible with respect to the real ion flow, the ionization manometer has a modulation factor k of 0.87.

The advantage of the high modulation factor in connection with a correction of desorption effects is clearly shown by the results obtained when oxygen is adsorbed at the grid-shaped anode. FIGS. 4 and 5 illustrate phenomena occurring in an oxygen-filled tube.

FIG. 4 illustrates the indicated pressure as a function of the modulationvoltage for the case in which a state of equilibrium is adjusted at a pressure of about 8X10" torr, while oxygen is supplied to the tube. The curves IV, V and VI are again associated with the values of the electron flow of 1 ma., 100m. and 10 a, respectively: they determine the following values of E:

After the supply of oxygen is cut off, the pressure drops rapidly to a value of about 10- torr and after the establishment of the equilibrium the result shown in FIG. 5 is obtained with an electron flow of 10 ,ua.

A comparison of the curve VII of FIG. 5, associated with a 75 value of 0.47, with the curve VI of FIG. 4, associated with a k value of 0.89, shows the strong influence of the desorption at the equilibrium pressure in the range of l0- torr. The variation of the indicated voltage plotted on the curve VII, which variation is small as compared with that plotted on the curve VI, is still such that the real ion flow and the residual current can be accurately determined from the result shown in FIG. 5. The accuracy would be considerably inferior on the basis of the results obtainable by a manometer having a modulation factor k of less than 0.5. In this case the characteristic curve corresponding with the curve VII would be substantially parallel to the axis on which the modulation voltage is plotted.

While the invention has been described 'with reference to a particularembodiment, other modifications will be apparent to those skilled in this art without departing from the spirit and scope of the invention which is defined in the appended claims.

What is claimed is:

1. In an ionization manometer, an elongated thin-wireshaped collector electrode, a grid-shaped anode surrounding said collector electrode, a filamentary cathode positioned external to the anode, terminal grids at opposite ends of the anode screening the interior space thereof, and means to apply a modulating voltage between at least one of the terminal grids, separated from the anode, and the anode.

2. An ionization manometer as claimed in claim 1, in which the collector electrode has a diameter of not exceeding a few microns.

3. An ionization manometer as claimed in claim 2, in which a difference potential is applied between the anode and the collector electrode which is about 3.5 times the potential difference between the anode and the cathode, and the maximum potential difference applied between the anode and the two terminal grids separated from the anode and the collector electrode.

References Cited UNITED STATES PATENTS 7/1968 Brock 3137 X 9/1869 Redhead 3137 US. Cl. X.R. 3137; 32433 75 3;" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3509418 Dated April 28, 1970 ANTONIUS GERARDUS JOHANNES VAN OOS'I'ROM Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 35, insert a "7" after -rods,-

SIGNED AND SFMEU AUG251970 (SEAL) Amst:

f mm: 1:. mm, 3 Amngom Domiasioner 0! Patents 

